1. Field of the Invention
The invention concerns a device for assembling and locking two tubes, in particular two tubes sliding one within the other.
It is more particularly concerned with a device for assembling and locking tubes in which locking is obtained by relative rotation of the tubes about a common axis of symmetry.
It also concerns the application of a device of this kind to the manufacture of a telescopic rod.
In the context of the invention, the expression "telescopic rod" must be understood in its widest sense: rod proper, sliding mast, etc.
By way of non-exhaustive example, the field of application of a telescopic rod of this kind concerns gardening. The telescopic rod may be fitted at one end with a tool of the secateurs or similar type, for example.
2. Description of the Prior Art
Many devices for assembling and locking two tubes of the above (rotational) type are known in themselves.
A first device known in itself comprises an eccentric cam and screw system. Upon relative rotation of the two tubes, the cam comes into contact with one of the tubes and frictionally engages its inside wall. However, this device is very complex and therefore costly and is of limited effectiveness. The contact area is small, resulting in a low coefficient of friction. Also, the cam being small, the mechanical strength is low.
Another known device comprises a screwthreaded central member surrounded by two half-cylinders. To be more precise, the screwthread is conical and entrains the two members when the central member is rotated about its axis of symmetry. The two half-cylinders are then moved apart angularly like two wings, and their ends frictionally engage the inside wall of one of the tubes.
FIGS. 1 and 2 appended to this description show a prior art device of this kind in diagrammatic form.
The assembly and locking device comprises a central member 1 in two parts: a screwthreaded front part 11 and a rear cylindrical sleeve 10. The latter fits in a first tube T.sub.1 sliding in a second tube T.sub.2 the inside diameter of which is slightly greater than the outside diameter of the first tube T.sub.1. The first tube T.sub.1 abuts on a ring 14 separating the screwthreaded front part 11 from the cylindrical rear sleeve 10. The two tubes T.sub.1 and T.sub.2 and the central member 1 have a common axis of symmetry .DELTA..
As already indicated, the screwthread 15 of the front part 11 is on a conical surface. The end of the screwthreaded front part 11 has an annular abutment 13 and the diameter of the screwthread 15 in this area is smaller than in the area near the annular abutment 14.
The screwthreaded front part 11 is surrounded by an assembly 2 comprising two contiguous half-cylinders 20 and 21. At the end near the cylindrical abutment 13 they have a peripheral groove 23 so that they may be retained by an elastic band 3 or similar means.
FIG. 1 shows the assembly and locking device in an initial state. In this state the two half-cylinders are abutted against the ring 13 and are inscribed within a cylinder having a diameter less than the inside diameter of the tube T.sub.2. Accordingly, the assembly can slide freely in a direction parallel to the previously mentioned axis of symmetry .DELTA..
In FIG. 1 the tubes T.sub.1 and T.sub.2 are shown in longitudinal section. The half-cylinder 20 is partly cut away in order to show more clearly how it cooperates with the screwthread 15. To be more precise, each half- cylinder has a raised pattern on its inside wall adapted to mesh with the screwthread 15. In FIG. 1 only the raised pattern 200 on the half-cylinder 20 can be seen.
As shown more particularly in FIG. 2, when a rotation R is imparted to the tube T.sub.1 the latter entrains the central member 1. It is assumed that the cylindrical sleeve 10 is forcibly fitted into the tube T.sub.1 and that the latter has fixing means, not shown, such as screws or similar means. The raised patterns associated with the two half-cylinders 20 and 21, for example the raised pattern 200, are entrained by the rotation imparted to the screwthread 15. It follows that the combination of the two half-cylinders 20 and 21 will move in translation towards the abutment 14, i.e. parallel to the axis of symmetry (arrow f). The raised patterns (for example 200) bearing on the surface of the screwthreaded area 11 and the latter itself being conical, the diameter increasing in the direction towards the abutment 14, the two half-cylinders 20 and 21 move apart at the end near the abutment 14. They are held close together at their other end by the elastic band 3.
It follows that the two half-cylinders 20 and 21 will come into contact with the inside wall of the tube T.sub.1 and apply pressure to it. This achieves the assembly and locking of the two tubes T.sub.1 and T.sub.2.
This device requires only low-cost, for example plastics material, parts, allowing manufacture by injection molding.
However, it is readily apparent from FIG. 2, which shows the device in the locked state, that the areas in which the bearing forces, and therefore the friction, are exerted are very small: end areas Z.sub.20 for the half-cylinder 20 and Z.sub.21 for the half-cylinder 21. It follows that the locking is somewhat unreliable. Furthermore, as the device relies on a buttress effect, there is a real risk of the components breaking if the two half-cylinders 20 and 21 are subjected to an excessively high force, i.e. if the relative rotation of the two tubes T.sub.1 and T.sub.2 is accentuated to increase the areas of contact.